Polybutylene terephthalate (PBT) resin is a well-known polyalkylene terephthalate that is semi-crystalline and has several desirable properties alone or in blends. Compared to amorphous resins such as acrylonitrile butadiene styrene (ABS), polycarbonate, and polystyrene, a semi-crystalline resin like PBT can show higher solvent resistance, strength, and stiffness due to the presence of crystalline spherulites in the resin. PBT resin is used in many applications in which its solvent resistance, strength, rigidity, and lubricity are needed, most commonly in durable goods that are formed by injection molding. Through its many blended products, PBT can be tailored to a wide variety of applications, including electronic and communications equipment, computers, televisions, kitchen and household appliances, industrial equipment, lighting systems, gardening and agricultural equipment, pumps, medical devices, food handling systems, handles, power and hand tools, bobbins and spindles, and automotive parts in both under-the-hood and exterior applications. Additionally, PBT is widely used to form electrical connectors.
It is generally known that PBT can be made by reacting 1,4-butanediol (BDO) with terephthalic acid (TPA) or dimethyl terephthalate (DMT) in the presence of a transesterification catalyst. U.S. Pat. Nos. 7,129,301; 6,020,393; 4,328,059, and US Patent Pub. 2005/0113534 A1 disclose various catalysts for the polymerization of polyesters.
Commonly used catalysts for the polymerization of PBT include tetraalkyl titanates. Among the various titanates are tetraisopropyl titanate, tetrabutyl titanate, and tetra(2-ethylhexyl) titanate. For example, JP 60147430 discloses a method of producing polyester by esterifying terephthalic acid, adipic acid and 1,4-butanediol in the presence of a titanium compound and a pentavalent phosphorus compound. U.S. Pat. No. 6,303,738 discloses a process for producing copolyester containing adipic acid in the presence of TYZOR® IAM (available from DuPont), which is prepared through the combination of tetraisopropyl titanate (TPT) and a mixture of butyl phosphate and dibutyl phosphate.
There are several factors that can lower the quality of the PBT resin. First, the catalyst used to prepare PBT is not typically quenched (deactivated) at the end of the polymerization process. Unfortunately, an active catalyst in the resin composition can sometimes lead to undesirable reactions in subsequent processing of the polyalkylene terephthalate. On exposure to high temperature and humidity, blends and compositions containing the polyalkylene terephthalate can exhibit hydrolytic degradation. Another problem associated with the presence of the active catalyst is transesterification, which can lead to loss of mechanical properties. Second, that the carboxylic end group concentration (CEG) of the PBT itself can have a negative impact on the hydrolytic stability of PBT resin. As the CEG of the PBT increases, the hydrolytic stability of the resulting resin decreases. Epoxy chain extenders are been used to rebuild molecular weight as hydrolysis occurs in PBT grades. However, in some instances, the addition of an epoxy chain extender does not succeed in rebuilding molecular weight and hydrolytic stability in the resulting PBT resin is not achieved.
As a result, a need remains for the discovery of processes for making hydrostable PBT resins.